Cavitation is the creation of gas bubbles around a propeller due to low water pressure, typically when the propeller is operating at high speed. A large amount of sound energy is produced as the gas bubbles almost immediately implode as water pressure normalises.
Cavitation noise can be heard from extreme distances and is usually avoided if at all possible. In C:MANO a ship or submarine that is cavitating will have 'CAV' displayed below its unit icon. The player can set ships and submarines to 'Avoid Cavitation' in the Speed / Altitude Settings Window (F2), but cavitation may still occur for short periods following rapid changes in depth.
When a submarines propeller turns quickly, areas of low pressure are created in the water surrounding the propeller blades. This lowering of pressure causes the water to boil without heating up, and produces bubbles of steam. This is called cavitation. The formation of these bubbles is based on propeller speed and ambient water pressure, with higher speed and lower pressure both tending towards cavitation. As depth increases, so does water pressure. For this reason to avoid cavitation at a certain speed, the solution is to dive deeper. If this is not possible, speed must be reduced in order for cavitation to cease. Cavitation is a dead give-away of the presence of a submarine which can be heard from extreme distances and most submarine commanders will only allow their submarine to enter cavitation parameters in dire emergencies.
To ensure that your crew automatically reduce speed to prevent cavitation, check the Avoid Cavitation box in the Throttle and Altitude settings window (F2). Even with this setting on, rapid changes in depth may result in short periods of cavitation while your submarine is slowing down to a speed below the new cavitation threshold.
Vessels that are cavitating will display CAV next to the unit icon.